Mass spectrometry (MS) is a powerful analytical technique that is used for the qualitative and quantitative identification of organic molecules, peptides, proteins and nucleic acids. MS offers speed, accuracy and high sensitivity. Key components of a mass spectrometer are the ion source, ion coupling optics, mass analyzer and detector. The ion source transforms analyte molecules into a stream of charged particles, or ions, through a process of electron addition or subtraction. The ions can be ‘steered’ using electric or magnetic fields. Ion coupling optics or lenses collimate the ion flux from the ion source into the mass analyzer. The analyzer separates ions by their mass to charge ratio. Several different kinds of mass analyzer are known in the art, including, but not limited to; magnetic sector, quadrupole, ion trap, time of flight and cycloidal. The ions exit the analyzer in order of mass to charge ratio and in so doing produces a mass spectrum which is a unique signature or ‘fingerprint’ for the analyte. Ions are directed to a detector where they impact and discharge an ion current which may be counted and amplified by signal electronics before being displayed on a computer screen as a mass spectrum. The detector is normally an electron multiplier. These components together form the analytical sub-systems of the mass spectrometer system. Other mass spectrometer system components include vacuum pumps, a vacuum chamber, drive electronics, data acquisition electronics, power supplies and enclosures.
It is sometimes necessary to analyse the interstellar dust or cosmic debris. For example, a number of mass spectrometer instruments have been constructed for space science purposes such as determining the composition of a comets tail, or to analyse interstellar dust particles, or to monitor the composition of the earth's ionosphere. In most cases these instruments have been based on an ionisation technique known as impact ionisation. There are several approaches used to analyze hypervelocity ions and particles in outer space. These methods were developed for various space science experiments and deployed as science payloads on space missions such as ‘Stardust’, ‘Genesis’, ‘Cassini’ and ‘Galileo’. The most reliable and efficient technique is kinetic impact ionization. In kinetic impact ionisation, a solid particle travelling at very high speeds impacts a solid target manufactured from a special material such as rhodium, gold, platinum, silver etc. The collision of the fast moving particle with the plate releases energy which partially ablates the target, but also generates a minute plume of plasma and ions. Each impact is fast and lasts a few femtoseconds. The ions so generated may be collimated and focussed into a mass spectrometer for analysis.
This method requires the detectable substance to be in the form of solid particles. In an exoatmospheric environment, such particles may be generated from cosmic bodies or man-made satellites. Liquid debris is likely to freeze into solid micro-droplets. Consequently, space debris from a liquid will convert into icy dust and will be suitable for impact ionization as well. The dust or ice particles of interest could have masses of between 10−14 kg and 10−17 kg and velocities of 1 km/s up to 10 km/s. These masses correspond to particles with a characteristic size of approximately 0.01 to 10 μm. A number of studies have demonstrated that ionization by kinetic impact is also feasible for liquid clusters of just 5,000 molecules (i.e. ˜7 nm size). Thus, impact ionization is likely to be an efficient means of ionising solid and liquid space debris.
The impact of a particle into a surface at high velocity (i.e. >1 km/s) resultant from kinetic impact ionisation (KII) produces vaporization and ionization of both the particle and a portion of the target. The ionization is traditionally held to result from the compression and heating at the point of impact. Various laboratory studies in the early 60's determined a correlation between the degree of ionization generated during an impact and the physical properties of the projectile (principally mass and velocity). In each of these studies the total charge, Q, produced during an impact fitted empirical relationship:Q=Kmpανβwhere K is a constant for each material (dependent largely on the atomic mass), mp is the mass of the particle, and is ν the impact velocity. Values for a range from 1.33 to 0.154 and seem to show a dependence on both the impact velocity and the experimental conditions. The value of β is usually near 1. The measurements show that for a given particle size an increase in velocity will produce a corresponding rise in the degree of ionization. A similar relationship can be observed experimentally for increasing particle mass. As the velocity and mass of a particle increases the energy released upon impact is greater.
A large proportion of the impact energy is lost in processes such as heating, melting and vaporizing the particle and target plate material. The energy fraction available to ionize the particle is in very small for low velocity impacts and rises to the order of a few percent for hypervelocity impacts. The efficiency with which the particle is ionized is also partially dependent on the incidence angle at which it strikes the target plate.
During kinetic impact ionization the particles of interest collide with the central element of the impact ionizer—the target ionization plate. The plate is usually a disk of a few centimeters in diameter and is made of a metal such as silver or rhodium. The particles have relative velocities of 1-10 km/s. The kinetic energy released in the collision with the ionization plate is sufficient to ionize the particle and some plate material. These ions are then focused into a mass spectrometer for analysis of their masses, traditionally using a time of flight (TOF) mass spectrometer.
Mass spectrometry is undoubtedly the best analytical technique for the analysis of space debris; it has unparalleled sensitivity, selectivity and the flexibility to determine the composition of a wide range of substances. All mass spectrometers are similar in that they can be broken down into six elements as shown in a schematic of the main elements of a typical mass spectrometer in FIG. 1:                A device to introduce a sample of the compound to be analysed—sample inlet        A source to generate ions from the sample—ion source        A mass analyzer to separate the ions according to their mass to charge ratio—mass analyzer        A detector to register the ions exiting the analyzer—detector        A computer to control the instrument and to process the data—computer        A means of relaying, communicating or displaying the mass spectral data—data display        
There are numerous design options for each of these mass spectrometer components. Established mass analysis techniques can be employed; the choice of which to use is influenced by many factors. The application and specificity of the final instrument is the biggest factor in determining which components are used to make-up the final system. The resulting system can be a highly flexible instrument capable of deployment in a wide range of roles, or a highly “specific to task” instrument unsuited to any other application.
Heretofore, KIIS has been used in the field of space science mostly for compositional and charge analysis of interstellar dust grains or comet particulates. The mass analyzer traditionally coupled with the KIIS for these particular types of space experiments is the “time of flight” mass analyzer or “TOF”. A simple linear TOF mass analyzer consists of a flight tube under vacuum at the end of which is an ion detector. The flight tube is held at ground if the ions are created at a positive potential, or if the ion source must be at ground, a liner is used within the flight tube and held at a potential equivalent to the ion acceleration potential. FIG. 2 depicts the principle of (linear) TOF mass separation.
TOF mass analyzers are based on a simple mass separation principle that two ionized species of different masses, with the same start point and time, accelerated by a homogenous constant electrostatic field will achieve velocities related to their mass to charge ratio. Their time of arrival at a detector will therefore directly indicate their masses. This principle is depicted in FIG. 2 and described below.
  t  =                    (                              2            ⁢            ms                    eE                )                    1        /        2              +                  D        ⁡                  (                      m                          2              ⁢                              eV                0                                              )                            1        /        2            where m is the mass of the particle, s is the length of the accelerating region, e is the electronic charge, E is the electrostatic field applied in the accelerating region, D is the length of the field free or ‘drift’ region and V0 is the accelerating potential.
In the ideal situation outlined above, given a long enough drift time or high enough accelerating potential, high resolution spectra can be achieved for a theoretically unlimited mass range, as the ions reach the detector in distinct ‘packets’. However in reality ions are not generated at one point in space and time. Variance within ion sources of initial temporal, spatial and energy distributions of the ions can widen peaks and reduce the resolution of the TOF-MS. In a densely packed ion source space charging can also occur, shielding ions and lowering their velocity as a consequence of the reduced accelerating potential experienced. The effect of spatial distribution is illustrated in FIG. 4. In FIG. 3(a) a packet of ions is formed in the ion source. They are all of the same m/z. In FIG. 3 (b), taking three individual ions with different initial spatial co-ordinates to represent the group, the ion nearest the extraction grid leaves first but as a consequence experiences the accelerating potential for a shorter time and has a lower kinetic energy. In FIG. 3 (c), the ion furthest away from the extraction grid leaves last but has a higher kinetic energy. In FIG. 3 (d) at a point in the drift region the faster moving ions will catch up with the slower ones. This is known as the primary focal point F. If a detector were to be located at this primary focal point the resolution would be very high. However in most instruments the primary focal point is only 100 mm beyond the ion source which is not enough flight time for mass peak separation. Therefore the result of putting a detector at point F would be very narrow but overlapping mass peaks. Therefore a longer flight time is necessary to get separation of ion masses but for the remaining flight time after point F the width of the ion packets is increasing, limiting the resolution.
The use of single and double stage reflectors can be used to enhance the resolution whilst increasing the flight time and using pulsed extraction and variable acceleration potentials can limit the spread of velocities of ions of identical m/z ratios leaving the source. These measures increase the flexibility and usefulness of the TOF-MS at the expense of simplicity of the system.
Advantages of the TOF mass analyzer coupled to the KIIS include:                High sensitivity        Theoretically unlimited mass range        Analysis Speed        Technology is field proven        
The disadvantages include:                Spectra could exhibit mass shifting        Secondary ionization effects        Ionization event length        Ion density within the ion source        Size and complexity of the instrument        Spectra obtained will be need to be post processed to obtain qualitative data        
The speed of analysis of the TOF-MS, its sensitivity and the fact that it can produce a complete mass spectrum for each sample particle impact appears to make it the perfect choice for a mass analyzer to be coupled to the KIIS. However, in the scenario where a very large amount of micro-particles must be analyzed in a very short space of time, the KIIS could experience a particle flux of approximately 1016 particles per second. This is a particle flux much greater than experienced by the TOF instruments previously used in space flight applications, where particle events were in the order of impacts per hour or even per week! Even those used to evaluate the dust particles in the coronas of comets did not experience such a high flux of particles. An ionization event lasting around 10 μs every 0.1 femtoseconds would overwhelm the TOF-MS capability to mass separate the produced ions due to peak broadening from multiple overlapping ionization events and secondary ionization events. Such a high flux would effectively ‘swamp’ a TOF-MS. Also the amount of ions within the source region could cause space charging further degrading the resolution.
A smaller impact plate area could reduce the particle flux and therefore would reduce the ‘swamping’ effect of a large number of impacts, but even reducing the impact plate area to 1 cm2 will only result in an order of magnitude reduction in particle flux. The size of a TOF instrument is also a problem, although there are some research groups developing miniature TOF-MS and Quadratic Field Reflection TOF instruments that may be small enough to be viable for the analyse of collision debris application, but of the fully developed instruments currently available none are of a small enough size to be contained in the limited space on an exoatmospheric payload for analysis of impact debris. Furthermore, TOF-MS does not scale well since as you miniaturise the instrument the flight path is shortened and the instrument resolution falls.
Because of these disadvantages, a TOF mass analyzer coupled with a KIIS is not a desirable solution for the analysis of the elemental and molecular composition of a very high flux of high velocity particles under exoatmospheric conditions, yet there is still a need for an analysis system that will allow for this analysis.